Since January 2004, Cambodia (estimated population 13 million) has experienced 25 epizootic outbreaks of H5N1influenza in poultry. In addition, 7 fatal human cases have been identified, including the death of a 13 year old patient during April 2007, who resided in Kampong Cham province. Following the unprecedented spread of H5N1, resources have been committed to better understanding avian and human influenza transmission and the epidemiology of viral respiratory infections in Cambodia. Until recently, there were relatively few disease surveillance systems that sought to capture disease with a high level of ascertainment and with laboratory confirmation of the etiology of the infection. There now exists in Cambodia a number of ongoing studies for acute febrile illnesses, including ones of dengue and influenza. These have resulted in the establishment of the infrastructure to conduct population-based surveillance at the household and health-care facility levels, and the capacity for screening, identification and testing of patients with influenza-like illness (III). To study the epidemiology of influenza in Cambodia we propose an approach that would build on several existing surveillance projects for influenza and non-influenza specific febrile illness and expand them with several proposed studies (Table below). Combined, the strengths of these study platforms and the data produced from them should provide a comprehensive and robust picture of influenza in Cambodia and address the specific objectives of this RFA. In addition, because the frequency of influenza related mortality is expected to be low, it is unlikely that the current surveillance studies would capture a significant number of influenza related deaths. To address this specific objective, and because death reporting is very poor in Cambodia, we would conduct verbal autopsies to estimate the death rate from III in an area where influenza has recently been documented. The specific aims are to: (1) Describe the clinical presentation and outcome of influenza, including the relationship of presentation to clinical case definitions, hospitalization rates and risk factors for severe adverse outcomes; (2) Describe the epidemiology of influenza in tropical, developing countries, including the seasonally, age distribution and transmission patterns; (3) Describe the burden of disease and health impact of influenza in tropical developing countries including incidence rates, hospitalization rates and mortality rates by age group. [unreadable] [unreadable] [unreadable]